Fluoropolymers have found use in many non-stick and release coating applications such as, for example, non-stick coatings for cookware. However, due to the non-stick nature of perfluoropolymers, primer layers having binders are generally needed to adhere to a surface of a substrate prior to application of the fluoropolymers, to enable sufficient adhesion of the fluoropolymer to the substrate. In the art, binder compositions comprising polyethersulfones (PES) or polyamideimides (PAI) or polyetherimides (PEI) or polyimides (PI), or combinations thereof, have been found to be useful in primer layers as film formers and for adhering to both the substrate surface and with perfluoropolymers.
Current binder compositions utilize N-methylpyrrolidone (NMP) as solvent due to its inexpense and solubilizing capabilities. See, for example U.S. Pat. No. 4,014,834 (to Concannon). More recently, the toxicity profile of NMP has been re-evaluated and pending regulations worldwide may minimize or eliminate the use of NMP in such binder solutions. Accordingly, alternative solvents, organic and aqueous based, are needed, which are considered to be more environmentally friendly. Suitable solvents will be capable of solubilizing the binders, including at practical concentrations or binder solids levels, allowing the binders to spread on the surfaces and create a primer layer or film on metal and rubber surfaces without adversely affecting the binder layer or its ability to bind to both the substrate and the fluoropolymer layers. The preferred solution to this problem would be to completely eliminate NMP from such binder solutions and present aqueous binder solutions. Unfortunately, these binder resins are completely insoluble in water. However, a water plus co-solvent approach may be an acceptable replacement for NMP, particularly if the aqueous binder solution is predominantly water, and the co-solvent has a much more favorable toxicity profile than NMP. The present invention meets these needs.